http://vimeo.com/groups/royaltyfree/videos
Vimeo is to video what Flickr is to images. This site is an online community for film makers, both professional and amateur. The site is free to use, both as a consumer and as a contributor. Vimeo does contain copyright free, royalty free, and creative commons videos. The site also puts limitations on content, making it a safe site for students to use.
I would use this site in a high school setting. This site is is a great place for students to post their presentations or to gather videos for the purpose of presentation. In my AP Language course, students create movie trailers each year depicting on work of Edgar Allan Poe. This would be a great site to post these videos for organizational purposes when presenting and for giving and receiving feedback with peers.
Potential problems with the use of this site, as with many others on the internet, is content. I have not explored all of the videos on the site. Though there are limitations on content, it is still important for the teacher to monitor students to assure that they are searching for and using appropriate material.

http://www.jamendo.com/en
This site offers royalty free, Creative Commons music created by artists and "given away" as a means of free promotion on the site. The site boasts some 390,000 songs, all of which can be played as a member or non-member. The site is easy to search by artist or by genre.
I an see this site being easy to use for middle or high school students. Because content is uploaded by the artists themselves, it is important to vigilant about making sure students censor their musical selections. I did notice some rather explicit material in some of the music that I would not want my students to be listening to in class. That being said, I would probably limit the use of this site to high school students who have a higher level of maturity.
Explicit material is the only potential problem I see with this site. The music on the site is very organized and the musicians very talented. This is not only a great site to use in presentations and video productions, but also for discovering great music.

Flickr is an online repository for images taken by users all over the world. The site is easy to navigate and free to join. The images on the site are royalty free, and the site operates under a Creative Commons License which only requires crediting the source of the image. The site could be used by just about any class at any age group. There is a search feature on the site that allows the user to search for images based upon tags attached to images by the contributor. Though the images on Flickr are taken by a variety of individuals with a variety of talent and experience levels, there are plenty of high quality and aesthetically pleasing photographs to choose from on the site.